Heavens to murgatroid! Peanut butter and orange juice? Deliberately tightening your vocal chords? Im having kittens just thinking about what is happening to your poor voice.
Here is some fundamental advice which applies to any type of singing:
1. Placement: Incredibly important. Correct placement is essential to get the feeling of producing the sound
out of your thoat, where it is most inhibitive. Yes, it is your vocal-chords that vibrate to produce sound, but this is not the place where you should be thinking that the sound comes from. Instead try this: Remember those playground taunts, Nya-nya! Try it now its a really nasal sound and it should seem like the sound is being produced right inside your nose (hence nasal.) Your lips should be buzzing, and probably tickle a bit.

You dont really want the sound in your nose for normal singing, but instead place it just behind your teeth, or imagine the sound is being produced just in front of your face. In your mid-range notes you should feel almost no sensation at all in your throat, and very little in your higher-range. Do not let the sound slip into your throat or try to push notes from your throat. It is an excercise in futility and you will wear your voice out very quickly. Gently humming up and down scales with correct placement is a basic warmup excercise.
2. Resonance. As I said before; although sound originates from vibrations in your vocal-chords, your voice is produced when those vibrating air-waves travel up into your mouth and start to resonate. The more space there is in your mouth, the louder the sound will be. You DO NOT need to strain in order to produce a loud sound. Indeed, singers who push from the throat make a restricted choking-sound to the listener, even though it seems to them as if they are being loud. As long as you have good resonance you will be loud. Lifting your soft-palette and keeping your tongue flat are the best ways to make space in your mouth for resonance. But do not think too hard about it, because you do not want to introduce tension into your mouth. For example, if you are singing fine but you think your tongue is in the wrong place; leave it. Do not introduce tension by trying to force it down.
3. Breathing & Support: Why you take a breath, imagine it is filling your stomach instead of your chest. This helps you to feel control in your diaphragm. Some schools of thought say that you should pull your stomach inwards when you are singing in order to control the note. It works for some people but personally I struggle with this. Instead, place your thumbs a few inches below your rib-cage on each of your sides. Push your thumbs inwards as if you were tickling yourself. At the same time use the muscles in your sides to resist your thumbs and push them away. You should also feel a tightness just below your ribcage on your tummy, and your tummy may roll slighty. You may also feel your back-muscles near where your kidneys are. Doing this prevents your diaphragm from rapidly collapsing in other words it gives you control of your breath. But do not worry about relaxing your sides as you sing out you dont need to get this technical. This will happen naturally as you run out of breath. Also, do not tense those support-muscles all the time, or you will exhaust yourself. Save support for the more demanding notes, such as sustained or high-notes.
4. Relax! Your worst enemy when singing is tension. You can do excercises to help loosen your jaw and mouth before warming up. One of this is simply grinning: Grin for a couple of minutes if you can. Then there is scrunching: Scrunch up your face and hold it for about 30 seconds, then open your mouth as wide as possible and very quickly. Do not do yourself injury

just unscrunch into a nice bright expression with an open mouth. Then there is brushing your teeth with your tongue. Move the tip of your tongue in a clockwise motion around the insides of your lips. Try and do this for a minute before taking a rest and doing it again anticlockwise. This one is a real bitch and your jaw-muscles will very likely be in agony the first time you try this, but it is great for loosening the jaw. Another jaw-excercise it to tilt your head upwards, then imagine you are trying to lift a full bucket with your jaw. You should feel your neck muscles tighten as you close your mouth and your jaw should be working too. Again, this is great for loosening the jaw. Be sure to test yourself with these excercises though: Overdoing it can introduce more tension instead of less, or just make you feel rough.
5. Relax! (Part 2) Going back to breathing again. When you get to the end of singing a phrase, think more about relaxing than of taking your next breath. If your tummy is tight from support (as it should be) relaxing it will drop your diaphragm and bring air inwards naturally, and without sound (ie, no gasping.) If you need more then take more, but be sure to relax as you do. Remember to relax your face at the same time just in case some tension has crept in as you were singing the last phrase.
6. Tuition. By far the best advice I can give is to get yourself a competent singing tutor who can listen to you and give you feedback on your singing. A good tutor will work with you to find the best methods of warming up your voice and helping you to sing in the style you want. It is entirely possible to sing with an edge or rock/metal sound without straining. With practice it is certainly possible to do so without peanut butter and orange juice.

Actually, it really does depend on how you want to sing. It is no good me getting Holier Than Thou saying you must not do this to your voice if the sound you want to make requires unorthodox technique. But do be aware that singers who frequently strain their voices with fundamentally poor technique will see the voices deteriorate as they get older. It is quite possible to sing with a growl, sound angry, and be able to do the same thing in 10 years time so long as you look after your voice.
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Well done if you managed to read all that.

If you only skimmed down to this part without reading, at least go back and read point 6. It is the most important point of all.
